For an object which is substantially at a constant distance to be watched repeatedly once a focus adjustment has been made, it is desired to maintain the optical system of a binocular at this focus adjusted position. Various techniques for this purpose have already been proposed. It is known, for example, to lock a rotatable member, operatively associated with back and forth movement of the optical system along the optical axes, temporarily at an angular position corresponding to the adjusted focus by use of a click-stop mechanism. It is also known to preselect a plurality of watching distances, for example, short-, intermediate- and long-distances, and to hold the optical system temporarily at any one of focus adjusted points corresponding to those watching distances, respectively.
However, these well-known techniques have been inconvenient in that the reliability for holding the focus-adjusted state is relatively low, the focus-adjusted state to be held often changes during repeated watching, or the reliable holding effect is achieved only by unacceptably troublesome operation. Accordingly, it has been desirable to solve these drawbacks simultaneously.